1. Department of Materials Science and Engineering Beckman Institute for Advanced Science and Technology, and Materials Research Laboratory University of Illinois at Urbana-Champaign Urbana Illinois 61801 USA
2. Present address: 3M Company 3M Center St. Paul MN 55144 USA
3. Department of Mechanical Science and Engineering University of Illinois at Urbana-Champaign Urbana Illinois 61801 USA
4. 4th Physics Institute and Research Center SCoPE University of Stuttgart Pfaffenwaldring 57 70569 Stuttgart Germany
5. Kirchhoff-Institute for Physics University of Heidelberg Im Neuenheimer Feld 227 69120 Heidelberg Germany